1. Field of the Invention
This invention relates to a supercharged engine, and more particularly to a supercharged engine in which a part of the pressurized air pressurized by a supercharger is introduced into the exhaust system as secondary air.
2. Description of the Prior Art
There is disclosed, in Japanese Patent Publication No. 59(1984)-5781, for instance, a supercharged engine in which a part of the pressurized air pressurized by a supercharger is introduced into the exhaust system as secondary air.
Generally, the supercharger must be able to discharge a large amount of air and the supercharging pressure in the supercharging passage must be high in order to ensure a sufficient amount of air at high speed and heavy load. On the other hand, feeding of the secondary air to the exhaust system is interrupted at high speed and heavy load in order to protect the exhaust system. That is, the secondary air is introduced into the exhaust system when the amount of supercharging air to the engine is zero or small. When the amount of supercharging air is small, an excessive amount of secondary air is apt to be fed to the exhaust system due to high supercharging pressure. In other words, for the purpose of feeding secondary air, the supercharger must be kept operated even in operating ranges in which supercharging is not effected and the pressure of the secondary air fed to the exhaust system cannot be sufficiently lowered even if excessive pressurized air is relieved through a relief passage. Therefore, high accuracy is required in controlling the flow of the secondary air.
If the amount of intake air is to be detected, e.g., in order to control fuel injection, for instance, by subtracting the amount of secondary air fed to the exhaust system from the total amount of air flowing into the intake passage through the air cleaner, the ratio of the amount of secondary air to the total amount becomes too large, when the amount of secondary air is excessive, to detect the amount of intake air with high accuracy, thereby adversely affecting fuel injection control.